Nearly three and a half years after its release, Windows 10 last month surpassed its enterprise predecessor, Windows 7, as the most popular operating system on the planet.

According to analytics vendor Net Applications, Windows 10 jumped by 1.1 percentage points in user share in December, climbing to 39.2% of all personal computers and 45.5% of all PCs running Windows. (The second number is larger than the first because Windows does not power all PCs; in December, Windows ran 86% of the world's systems. Most of the remainder ran macOS, Linux or Chrome OS, in decreasing order.)

Meanwhile, Windows 7 lost 2 percentage points last month, four times the loss of November and the largest one-month decline in over a year. Windows 7 ended 2018 on 36.9% of all personal computers and on 42.8% of all Windows PCs.

The moment when Windows 10 replaced Window 7 - the crossover point - came a month later than Computerworld predicted two months ago. Using the average monthly movement of each OS, that forecast envisioned a November crossover. It didn't happen. (To be honest, the predictions were regularly voided when Windows 10 failed to meet growth expectations; in January 2018, for instance, crossover was tapped to occur in August.)

Crossover comes late

Crossover came late to Windows 7. The last major migration - from Windows XP to Windows 7 - reached crossover in August 2012, or 20 months before XP's April 2014 retirement. Windows 10's was less than 13 months before Windows 7's retirement, currently slated for Jan. 14, 2020.

When Windows 7's support ends, it should be powering slightly more than 36% of all Windows PCs, while Windows 10 will be running 55%. The first number - the projected user share for Windows 7 at its retirement - fell from the month-ago forecast (which pegged it at a record 40%) because the 2009 OS returned to a large decline in December.

By comparison, Windows XP accounted for 29% of all Windows PCs when it dropped off the support list.

Six months after Windows 7's support expiration date, the OS should still power between 32% and 33% of all Windows PCs. By then, Windows 10 will have climbed to approximately 59%. In plainer terms, there will be lots of personal computers running the then-outdated Windows 7 without the safety net of security updates.

Microsoft will offer a temporary solution to businesses running Windows 7 Professional or Windows 7 Enterprise - the post-retirement Windows 7 Extended Security Updates (ESU) announced in September 2018 - for an escalating cost each year for up to three years.

Windows 10 adoption has been slower than 7's

Even with Microsoft's free Windows 10 upgrade offer for the first 12 months after launch, the operating system was never able to keep up with the adoption pace set by Windows 7 six years earlier.

In the first three complete calendar years after Windows 7's debut - 2010, 2011 and 2012 - the OS added 16, 15 and 8 percentage points to its total, respectively, for a total of 39 points of user share.

However, Windows 10 added just 14, 9 and 6 percentage points - a total of 29 - during its three-year stretch of 2016, 2017 and 2018. Windows 10 never matched the growth of its ancestor, whether on a one-year basis or overall. And compared to Windows 7, Windows 10 was unable to sustain high volumes of growth: Windows 10's slowed dramatically in the second year and its third year also fell short of Windows 7's.

Bottom line: Net Applications' data points to a mad dash to rid systems of Windows 7 just before, at and after the support deadline, one even more frantic than took place in the months before and after Windows XP's end of support.

Elsewhere in December's data, the user share of Windows slumped once again, dropping eight-tenths of a percentage point - the most since November 2017 - to 86%. The combined share of all macOS and OS X editions added nine-tenths of a point to the total user share, which ended the year at 10.7%, a record high for Apple's OS. Linux's user share rose by seven-tenths of a point and Google's Chrome OS stayed flat, with those alternate OSes at 2.8% and 0.3%, respectively.

Net Applications calculates user share by detecting the agent strings of the browsers people use to visit its clients' websites. It then tallies the visitor sessions. The result: a measurement of browser activity that reflects OS use.